Breast cancer kills over 40,000 women every year in the US alone, yet preventive strategies are predominantly restricted to anti-estrogenic approaches with their attendant limitations. We hypothesize that beta-catenin/TCF signaling may be a novel target for breast cancer prevention. Aberrant activation of the Wnt/beta-catenin signaling pathway is widespread in human cancers, and is known to be an initiating event in human colorectal carcinogenesis. In marked contrast, the role of beta-catenin/TCF signaling in breast cancer is poorly understood. Nucleocytoplasmic beta-catenin has been observed in a high proportion of invasive breast carcinomas. However, the mechanism of beta-catenin stabilization has not been established. Furthermore, the activation status of this pathway in precancers of the breast has not been determined. Our preliminary studies have revealed evidence of beta-catenin/TCF pathway activation in breast hyperplasias and ductal carcinoma in situ (DCIS) lesions, frequent precursors to invasive breast cancer. Additionally, we have identified a relationship between HER2/neu (human epidermal growth factor receptor 2) and beta-catenin/TCF signaling. This is of particular importance because HER2/neu is overexpressed in 50-60% of human DCIS precancers. Therefore, the goal of this research is to test the role of beta-catenin/TCF signaling in early breast neoplasia. In particular, we aim to define the activation status of this pathway in breast precancers, and to test the requirement for beta-catenin/TCF signaling for breast tumor development. Three specific aims are proposed to achieve these goals. AIM 1 is designed to determine the extent of beta-catenin/TCF signaling pathway activation in precancerous breast lesions and invasive breast cancers, using beta-catenin immunohistochemistry and TCF reporter mouse strains. AIM 2 will interrogate the molecular mechanisms by which HER2/neu activates this pathway. In AIM 3, a dominant negative suppressor of beta-catenin/TCF- dependent transcription will be used to test the requirement for beta-catenin/TCF signaling for breast tumor formation in vivo. The realization that aberrant activation of Wnt/beta-catenin signaling is highly prevalent in human neoplasia has stimulated enormous interest in developing beta-catenin/TCF antagonists as cancer therapeutic agents. Positive results in our study should pave the way for testing the efficacy of such beta-catenin/TCF antagonists for breast cancer prevention. Important in this respect is the predicted relationship between HER2/neu and beta-catenin/TCF signaling in DCIS, since this would allow specific HER2- directed targeting of beta-catenin/TCF antagonists to breast precancers. Our research is an essential first step in the development of beta-catenin/TCF signaling as a novel target for the prevention of progression of precancerous breast lesions to invasive breast cancer. PROJECT NARRATIVE RELEVANCE Over 40,000 women die from breast cancer every year in the US alone, and yet our arsenal of breast cancer prevention strategies is woefully inadequate. The goal of this research is to evaluate beta-catenin/TCF signaling as a novel target for breast cancer prevention. The beta-catenin/TCF signaling pathway is frequently activated in human cancer, and is known to be particularly important in colorectal cancer. In contrast, the role of this pathway in early breast carcinogenesis is poorly understood. We propose to examine the role of beta-catenin/TCF signaling in early breast cancer. Firstly, we will determine the extent of beta-catenin/TCF signaling pathway activation in precancerous breast lesions and invasive breast cancers. Secondly, we will test the mechanism by which this pathway is activated. Thirdly, we will test the requirement for this pathway for breast tumor formation in vivo. Importantly, beta-catenin/TCF antagonists are already in development as cancer therapeutic agents. Positive results in our study would lay the foundation for evaluating the utility of such beta-catenin/TCF antagonists for preventing human breast cancer.